Hydrogen peroxide (H2O2) is a versatile commodity chemical with diverse applications. Hydrogen peroxide applications take advantage of its strong oxidizing agent properties and include pulp/paper bleaching, waste water treatment, chemical synthesis, textile bleaching, metals processing, microelectronics production, food packaging, health care and cosmetics applications. The annual U.S. production of H2O2 is 1.7 billion pounds, which represents roughly 30% of the total world output of 5.9 billion pounds per year. The worldwide market for hydrogen peroxide is expected to grow steadily at about 3% annually.
Various chemical processes may be employed to manufacture hydrogen peroxide on a commercial scale. One major class of hydrogen peroxide manufacture comprises the auto-oxidation (AO)(also called autoxidation) of a “working compound” or “working reactant” or “reactive compound”, to yield hydrogen peroxide. Commercial AO manufacture of hydrogen peroxide has utilized working compounds in both cyclic and non-cyclic processes.
The cyclic AO processes typically involve hydrogenation (reduction) of a working compound and then auto-oxidation of the hydrogenated working compound to produce hydrogen peroxide. Most current large-scale hydrogen peroxide manufacturing processes are based on an anthraquinone AO process, in which hydrogen peroxide is formed by a cyclic reduction and subsequent auto-oxidation of anthraquinone derivatives. The anthraquinone auto-oxidation process for the manufacture of hydrogen peroxide is well known, being disclosed in the 1930s by Riedl and Pfleiderer, e.g., in U.S. Pat. No. 2,158,525 and No. 2,215,883, and is described in the Kirk-Othmer Encyclopedia of Chemical Technology, 3rd. ed., Volume 13, Wiley, New York, 1981, pp. 15-22.
In addition to the anthraquinones, examples of other working compounds feasible for use in the cyclic auto-oxidation manufacture of hydrogen peroxide include azobenzene and phenazine; see, e.g., Kirk-Othmer Encyclopedia of Chemical Technology, 3rd. ed., Volume 13, Wiley, New York, 1981, pp. 15 & 22.
In commercial AO hydrogen peroxide processes, the anthraquinone derivatives (i.e., the working compounds) are usually alkyl anthraquinones and/or alkyl tetrahydroanthraquinones, and these are used as the working compound(s) in a solvent-containing working solution. The anthraquinone derivatives are dissolved in an inert solvent system. This mixture of working compounds and solvent(s) is called the working solution and is the cycling fluid of the AO process. The solvent components are normally selected based on their ability to dissolve anthraquinones and anthrahydroquinones, but other important solvent criteria are low vapor pressure, relatively high flash point, low water solubility and favorable water extraction characteristics.
Non-cyclic AO hydrogen peroxide processes typically involve the auto-oxidation of a working compound, without an initial reduction of hydrogenation step, as in the auto-oxidation of isopropanol or other primary or secondary alcohol to an aldehyde or ketone, to yield hydrogen peroxide.
Hydrogenation (reduction) of the anthraquinone-containing working solution is carried out by contact of the latter with a hydrogen-containing gas in the presence of a catalyst in a large scale reactor at suitable conditions of temperature and pressure to produce anthrahydroquinones. Once the hydrogenation reaction has reached the desired degree of completion, the hydrogenated working solution is removed from the hydrogenation reactor and is then oxidized by contact with an oxygen-containing gas, usually air. The oxidation step converts the anthrahydroquinones back to anthraquinones and simultaneously forms H2O2 which normally remains dissolved in the working solution.
The remaining steps in conventional AO processes are physical operations. The H2O2 produced in the working solution during the oxidation step is separated from the working solution in a water extraction step. The working solution from which H2O2 has been extracted is returned to the reduction step. Thus, the hydrogenation-oxidation-extraction cycle is carried out in a continuous loop, i.e., as a cyclic operation. The H2O2 leaving the extraction step is typically purified and concentrated. An overview of the anthraquinone AO process for the production of hydrogen peroxide is given in Ullman's Encyclopedia of Industrial Chemistry, 5th Edition, Volume A13, pages 447-456.
The productivity and selectivity of the hydrogenation reaction are critical to the economics of the AO process. Further, hydrogenation catalyst life and catalyst attrition have strong influences on the ease of operation, safety and economics of the AO process. Additionally, the auxiliary catalyst separation equipment required downstream of the hydrogenation reaction can be costly depending upon which type of hydrogenation reactor is chosen.
Conventional catalytic hydrogenation reactors give rise to undesirable hydrogenated byproduct compounds such as oxanthrones and anthrones. These compounds ultimately degrade into reactants no longer capable of producing hydrogen peroxide. The rate of generation of these parasitic byproducts has a strong influence on the sustained capacity and operating costs of a conventional AO process.
The hydrogenation reaction in traditional cyclic AO hydrogen peroxide processes is conventionally carried out in a large scale fluid-bed or fixed-bed reactor. The fluid-bed reactor is also commonly known as the suspension catalyst reactor. The fixed-bed reactor is used in a variety of different types including trickle-bed reactors, dispersed- and foam-flow reactors, and the monolithic catalyst structure reactor to name just a few. Each type has its advantages and disadvantages.
Fluid-bed hydrogenation reactors achieve good contact among the three phases and, thus, obtain high activity and selectivity. However, costly and technically elaborate solids removal devices (e.g., filters, cyclones, etc.) are required to prevent the catalyst from entering the oxidation stage of the AO process where the catalyst can lead to unsafe decomposition of hydrogen peroxide. In the fluid-bed reactor, the catalyst is susceptible to abrasion over time and, thus, fines can be produced which can further complicate the elaborate downstream solids separation devices. Furthermore, the expensive catalyst is underutilized since a portion is maintained inside these elaborate catalyst separation devices instead of directly inside the reactor.
The fluid-bed hydrogenation reactor scheme can take on one of several different types. The reactor can be a stirred-type reactor or achieve its mixing based on the airlift principle (GB Patent 718,307). The fluid bed scheme can alternatively involve tubular reactors (U.S. Pat. No. 4,428,923) in which mixing is achieved by turbulence from high flow velocity. Such tubular reactors can also achieve mixing by adjustments in the tube diameter (U.S. Pat. No. 3,423,176), by premixing the hydrogen and working solution with static mixers (U.S. Pat. No. 4,428,922), or by premixing the reaction mixture and hydrogen with venturi nozzles (U.S. Pat. No. 6,861,042).
The fixed-bed reactors do not abrade the catalyst to the same degree as the fluid bed reactors. Since the catalyst is in a fixed, stationary position within the reactor, the fixed-bed reactors require much simpler, less costly downstream catalyst separation devices. Further, the fixed-bed reactors do not result in back mixing if operated in a co current flow pattern and, thus, are capable, at least in theory, of higher volumetric productivities owing to the near plug flow. However, productivity and selectivity results are often poorer than those obtained with fluid-bed reactors due to uneven flow distribution and/or excessive bed pressure drop. Lastly, fixed-bed reactors normally require stoppage to remove deactivated catalyst unless costly parallel reactor trains are installed.
Another embodiment (U.S. Pat. No. 5,637,286) of a fixed-bed reactor employs foam like mixtures of the working solution and hydrogen with the aim of increasing the productivity. The reported productivity is 172 kg H2O2/(h-m3) where m3 is the volume of the catalyst bed. However, the flux of the working solution required for this reactor arrangement results in high pressure drop and high energy expenditure. Further, the catalyst is more susceptible to abrasion due to the high work solution flux.
Fixed-bed reactors employing monolithic catalyst beds (U.S. Pat. No. 4,552,748 and No. 5,063,043), commonly called honeycomb structure, seek to overcome some of the disadvantages of the previous fixed-bed reactors by ensuring uniform contact time, lower pressure drop, high selectivity, and efficient palladium utilization. In a honeycomb structure reactor (U.S. Pat. No. 5,063,043), the reported yield is 133 kg H2O2/(h-m3) where m3 is the volume unit of the reactor structure. However, even distribution of the liquid is problematic and these honeycomb structured catalyst elements require a complicated production technique, as is discussed in U.S. Pat. No. 5,637,286.
All of AO process hydrogenation reactors have the disadvantage in safety in that they contain large reactant inventories. A key safety concern in the AO process is preventing sufficiently high levels of unextracted hydrogen peroxide exiting the extraction stage from entering the hydrogenation stage, upon recycle of the working solution. All unextracted hydrogen peroxide entering the hydrogenation stage is a source of oxygen, and explosive hydrogen/oxygen mixtures can result if unextracted hydrogen peroxide levels are not properly controlled. Because all AO process hydrogenation reactors employ large reactant inventories, the consequences of a hydrogen/oxygen explosion in the hydrogenation reaction can be very severe.
A very different approach for avoiding the drawbacks associated with hydrogenation reactions in large-scale AO processes is the direct synthesis of hydrogen peroxide from reaction of hydrogen and oxygen, which eliminates the separate hydrogenation and oxidation steps. One such direct synthesis process is described in U.S. Pat. No. 7,029,647, in which the staged reaction of the hydrogen and oxygen reactants is carried out in a microchannel reactor.
It is a principal object of this invention to provide an improved method for the hydrogenation stage of a conventional AO process for producing hydrogen peroxide while maintaining the advantages of a fixed bed hydrogenation reactor over a fluid bed reactor and maintaining high productivity and selectivity, high hydrogen utilization per pass, and high-grade waste heat of reaction. Another object of this invention is to provide a safer process for the hydrogenation stage by employing reduced reactant inventories over the conventional AO hydrogenation reactors.
The present invention achieves these and other objectives in the auto-oxidation production of hydrogen peroxide, using a hydrogenation stage carried out in a microreactor.